Demands for solid-state imaging devices are recently rapidly increasing for use in digital still cameras and video cameras to input images. Charge-coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors are widely used as solid-state imaging elements used in the solid-state imaging devices described above.
A solid-state imaging element is configured to comprise photodiodes which convert optical signals into electric signals, and electrically read an image projected on an imaging plane. A solid-state imaging device comprises an analog to digital (A/D) converter which converts electric signals (analog voltages) read by the solid-state imaging element into digital signals.
Conventional A/D converters for solid-state imaging elements operate in a manner as follows. At first, a reference voltage for a triangular wave is generated by a constant current source and an integrator. Subsequently, an analog voltage and a reference voltage are compared with each other by a comparator, to detect a time point when values thereof become equal. This time point is measured by a timer, an output of which is converted from an analog signal into a digital signal.
In this manner, however, the reference voltage is generated starting from 0 V as a start point.
Therefore, if an analog voltage to be subjected to digital conversion is high, a long time is required. Reading needs be performed at a data read speed of the imaging element, which is lowered so as to correspond to a conversion time for a maximum voltage. Therefore, a reading speed of a solid-state imaging device which combines a solid-state imaging element and an A/D converter is limited by a conversion speed of the A/D converter. Reading is difficult at a higher data transfer speed (data rate) than the conversion speed thereof.